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<h1>Java HDF5 Interface (JHI5)</h1>

<h2><u>What it is</u></h2>
The <b>Java HD5 Interface (JHI5)</b> is a Java package
(<a href="./hdf/hdf5lib/package-summary.html">hdf.hdf5lib</a>)
that ``wraps around'' the HDF5 library.
<p>There are a large number of functions in the HDF5
library (version 1.14). Some of the functions are not supported in JHI5. Most
of the unsupported functions have C function pointers, which is not currently
implemented in JHI5.</p>

<table style="width:71%;text-align:center;background-color:#dbeaf5" border="1">
<caption>Java HDF5 Interface (JHI5)</caption>
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<td style="text-align:center;">
Note: The JHI5 does not support HDF4 or earlier.
</td>
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<p>The JHI5 may be used by any Java application that needs to access HDF5
files. It is extremely important to emphasize that <i>this package is not
a pure Java implementation of the HDF5 library.</i> The JHI5 calls the
same HDF5 library that is used by C or FORTRAN programs. (Note that this
product cannot be used in most network browsers because it accesses the
local disk using native code.)
<p>The Java HDF5 Interface consists of Java classes and a dynamically
linked native library. The Java classes declare native methods, and the
library contains C functions which implement the native methods. The C
functions call the standard HDF5 library, which is linked as part of the
same library on most platforms.
<p>The central part of the JHI5 is the Java class <i>
<a href="./hdf/hdf5lib/H5.html">hdf.hdf5lib.H5</a></i>.
The <i>H5 </i>class calls the standard (<i>i.e.</i>, `native' code) HDF5
library, with native methods for most of the HDF5 functions.

<h3>
<u>How to use it</u></h3>
The JHI5 is used by Java classes to call the HDF5 library, in order to
create HDF5 files, and read and write data in existing HDF5 files.
<p>For example, the HDF5 library has the function <b>H5Fopen</b> to open
an HDF5 file. The Java interface is the class <i>
<a href="./hdf/hdf5lib/H5.html">hdf.hdf5lib.H5</a></i>,
which has a method:
<pre><b>static native int H5Fopen(String filename, int flags, int access );</b></pre>
The native method is implemented in C using the
<a href="https://docs.oracle.com/en/java/javase/18/docs/specs/jni/index.html">Java
Native Method Interface </a>(JNI). This is written something like the following:
<pre><b>JNIEXPORT jlong
JNICALL Java_hdf_hdf5lib_H5_H5Fopen
(
&nbsp;JNIEnv *env,
&nbsp;jclass class,
&nbsp;jstring hdfFile,
&nbsp;jint flags,
&nbsp;jlong access)
&nbsp;{

&nbsp;/* ...convert Java String to (char *) */

&nbsp;/* call the HDF library */
&nbsp;retVal = H5Fopen((char *)file, (unsigned)flags, (hid_t)access);

&nbsp;/* ... */
}</b></pre>
This C function calls the HDF5 library and returns the result appropriately.
<p>There is one native method for each HDF entry point (several hundred
in all), which are compiled with the HDF library into a dynamically loaded
library (<i>libhdf5_java</i>). Note that this library must be built for each
platform.
<p>To call the HDF `<b><i>H5Fopen</i></b>' function, a Java program would
import the package '<i><b>hdf.hdf5lib.*</b>'</i>, and invoke the method
on the class '<b><i>H5</i></b>'. The Java program would look something
like this:
<pre><b>import hdf.hdf5lib.*;

{
&nbsp;/* ... */

&nbsp;try {
&nbsp;file = H5.Hopen("myFile.hdf", flags, access );
&nbsp;} catch (HDF5Exception ex) {
&nbsp;//...
&nbsp;}

&nbsp;/* ... */
}</b></pre>
The <i><b>H5</b> </i>class automatically loads the native method implementations
and the HDF5 library.

<h3>To Obtain</h3>
The JHI5 is included with the <a href="https://support.hdfgroup.org/downloads/index.html">HDF5</a> library.

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